A gas diffusion electrode allows a gas such as hydrogen, oxygen, or air to be supplied to a porous electrode and to react on the electrode. The gas electrode is used in a fuel cell, a metal-air battery or the like which converts the chemical energy of gas to electric energy.
In the field of brine electrolysis, a gas diffusion electrode in the form of a cathode that is capable of realizing energy-saving is being developed. This development is under way because a cathodic reaction that can change from a current hydrogen generation reaction to an oxygen reduction reaction will produce a significantly reduced electrolysis voltage.
Various gas diffusion electrodes are known for specific applications. The gas diffusion electrode, which is known to use an electrolyte solution in the form of an aqueous solution, is a laminated structure between gas diffusion and reaction layers, in the inner part of which a collector for electrical connection is embedded. Oxygen is supplied from the side of the gas diffusion layer, and the reaction layer is in contact with the electrolyte. After permeating the gas diffusion layer for diffusion in the inside, oxygen is subjected to a reduction reaction on an oxygen reduction catalyst fixed in the reaction layer.
Methods for brine electrolysis are described below, which use a current hydrogen cathode system, and an oxygen cathode system employing the gas diffusion electrode. A hydrogen generation reaction on the hydrogen cathode, an oxygen generation reaction on the gas diffusion electrode, and a chrorine generation reaction on an anode are represented by formulas 1, 2, and 3, respectively.2H2O+2e−→2OH−+H2: electrode potential; −0.828 V  (Formula 1)O2+2H2O+4e−→4OH−: electrode potential; 0.401 V  (Formula 2)2Cl−→Cl2+2e−: electrode potential; 1.36 V  (Formula 3)
Further, Na+ penetrates an ion exchange membrane and moves from an anode chamber into a cathode chamber. When combining these, the overall reactions in the hydrogen cathode and oxygen cathode systems are represented by formulas 4 and 5, respectively.2NaCl+2H2O→2NaOH+Cl2+H2  (Formula 4)2NaCl+H2O+1/2O2→2NaOH+Cl2  (Formula 5)
The theoretical electrolysis voltage that corresponds to the difference in voltage between cathodic and anodic reactions requires 2.19 V for the hydrogen cathode system, while 0.96 V is needed for the oxygen cathode system, allowing an electrolysis voltage of 1.23 V to be decreased. However, the merit of the decrease of 1.23 V can not be totally enjoyed e.g. because: oxygen must be supplied as a raw material; hydrogen cannot be obtained as a product; and an overvoltage for oxygen reduction reaction is now a larger voltage than that for hydrogen generation reaction.
Previously known catalysts which are high in the activity to reduce oxygen (hereinafter referred to as oxygen reduction activity) include platinum, silver, organometallic complexes, perovskite oxides, or the like, as described, for example, in JP-A-2000-212788; JP-A-02-257577; JP-A-07-289903; and F. C. Anson, et al., J. Am. Chem. Soc., 1980, 102, 6027. These catalysts mainly use a particulate carbon as a carrier, and are supported on it in a highly dispersed condition. However, their catalytic activities are not sufficient, and use of them as a cathode leads to elevated overvoltages. As a result, the oxygen cathode system cannot enjoy the documents of the current hydrogen cathode system, in view of the costs including the oxygen cost. Thus, there is a need for a catalyst with higher oxygen reduction activity in the art.
Attempts have been made to use rare-earth oxide in combination with various metals and oxides as an electrode for an electrode catalyst for oxygen reduction, a fuel cell, or the like. In JP-A-2003-100308, a catalyst for oxygen reduction for polymer electrolyte fuel cell applications has been proposed in which platinum or platinum-molybdenum alloy and cerium oxide are supported on carbon. In addition, electrodes for an oxygen pump cell and the like, obtained by forming, into electrodes, mixtures of various elemental metals or their alloys and rare-earth elements or their oxides and an electrochemical cell are disclosed in JP-A-2002-333428.
In solid oxide fuel cell applications, examples are further disclosed in which a highly dispersed mixture of a metal powder of nickel, platinum, or ruthenium and a cerium oxide powder disclosed in JP-A-11-297333, and a highly dispersed mixture of a perovskite oxide powder and a cerium oxide powder found in JP-A-11-214014 have been formed into electrodes. In JP-A-11-297333 and JP-A-11-214014, samarium and the like are allowed to form a solid solution in crystalline cerium oxide for enhancing the durability and the oxide ion conducting properties of an electrode.
However, it is difficult to obtain excellent performance by applying the catalyst for oxygen reduction and the electrode described above to a gas diffusion electrode for brine electrolysis or a metal-air battery. The gas diffusion electrode and the electrode catalyst for oxygen reduction which are used in brine electrolysis applications require excellent resistance to alkalis and to oxidation because they are employed in an aqueous caustic soda solution with a concentration of 30 wt % or more, thus, differing from a conventional hydrogen cathode, in an oxidizing atmosphere.
For example, the composite catalyst of platinum-molybdenum alloy and cerium oxide exhibiting the highest oxygen reduction activity in JP-A-2003-100308 is expected to produce eluted molybdenum with time in a concentrated alkaline atmosphere in which brine electrolysis is effected, generating reduced activity. It has been also described that the perovskite oxide used in JP-A-11-214014 has a high catalytic activity for oxygen reduction in an alkaline atmosphere. However, at present, the durability thereof represents a large problem for the development since it is decomposed over time.
In electrodes where various elemental metals or their alloys and elemental rare-earth or its oxide have been made composite, as described in JP-A-2002-333428 and JP-A-11-297333, the powder is prepared by mechanical mixing and evaporative decomposition and has a substantial particle diameter of a few micrometers. In the gas diffusion electrode, it is essential to form a three-phase interface consisting of oxygen gas, a catalyst for oxygen reduction, and an electrolyte solution so that the interface has a sufficiently large area, and therefore a particulate carbon of 1 μm or less is load with finer particles of electrode catalyst thereon, the constitution, structure and effective surface area of the electrode being much different from those in the above references.